1. Field of the Invention
The invention relates generally to lamp controllers for controlling lamps having filament electrodes, such as low pressure mercury vapor fluorescent lamps. More particularly, the invention concerns circuitry in such a lamp controller which detects lamp status to implement various safety features.
2. Description of the Prior Art
Low pressure mercury vapor fluorescent lamps exhibit negative resistance characteristics, that is lamp resistance decreases after ignition. A lamp controller or ballast provides a high starting voltage between the lamp electrodes and controls the lamp operating current after starting. In rapid start systems, the lamp electrodes are filaments and filament current is provided to heat the filaments, which lowers the voltage required for lamp starting.
It is common for ballasts to control two or more lamps. A disadvantage of many known ballast designs is that if a filament electrode of one of the lamps fails, or if a lamp is removed from its socket, the ballast will sense that a lamp has extinguished and will attempt to reignite the failed or removed lamp. This is accomplished in some ballasts by applying a high starting voltage across all of the lamps for a short period, and repeating until the ballast senses that all lamps are lit. This starting cycle results in a very undesirable flashing of the remaining working lamp(s). This flashing causes a decrease in lamp life because it stresses the filament electrodes unnecessarily. It also presents a shock hazard in that high voltages are present at the lamp socket contacts. The shock hazard relative to ground can be substantially avoided with an isolation transformer, but a shock hazard across the socket contacts at the opposite end of a lamp or set of lamps would still exist.
U.S. Pat. No. 4,382,212 (Bay) discloses an electronic ballast having a push-pull inverter and which includes a shut down circuit for detecting the removal of at least one lamp filament from its respective socket, or failure of a filament, and for stopping the operation of the high frequency inverter in such event. The shut down device includes a pair of feedback windings each to provide a feedback signal from a lamp filament current and a differential transformer having a first winding coupled to one feedback winding and a second winding coupled to the other feedback winding. During normal operation, the flux in the core of the differential transformer due to the current in the first and second windings will cancel each other. When one of the filaments burns out or is removed, the flux cancellation will be upset. The resulting impedance of the differential transformer is used to shut down oscillation of the high frequency inverter through a feed-back winding input of the push-pull circuit. A disadvantage of this arrangement is that it requires a differential transformer in addition to the primary transformer, and hence increases circuit cost.
U.S. Pat. No. 4,952,849 (Fellows et al) discloses a ballast having an inverter coupled to the load by a resonant output circuit. The output circuit produces an ignition voltage at an inverter frequency offset in one direction from its resonant frequency. Upon energization of the ballast, the lamp filaments are first preheated with high frequency currents to aid ignition and the controller then sweeps the inverter frequency from a high level above that at which ignition could occur to a lower level at which the ignition voltage is produced. After ignition, stable operation is achieved in a frequency range well above the resonant point of the output circuit. During the preheat and ignition phases of operation, a lamp voltage regulator circuit limits the maximum open circuit voltage across the lamps. If the lamp does not ignite after a preselected number of attempts or if a lamp fails, the lamp voltage regulator circuit initiates a re-ignition operation in which the operating frequency is rapidly switched to its maximum value and then gradually reduced to increase the operating voltage, to thereby make another attempt at lamp ignition. However, this has the adverse affect discussed above of continuously "flashing" the remaining lamp, as well as providing a high ignition voltage at the lamp socket even if a lamp is not present. The ballast also provides an output voltage upon initial energization of the ballast even if no lamps are present.
U.S. Pat. No. 5,089,753 (Mattas) discloses a form of protection circuitry for the ballast of the Fellows '849 patent. The protection circuit includes a bridge arrangement which senses the current flow through each of two lamps in a two lamp system and limits the number of ignition attempts in the event that one of the lamps ignites and the other doesn't. This limits the duration of the flicker effect caused by only one lamp being lit during repeated ignition attempts. If one of the lamps does not ignite, a capacitor is charged so that after a selected number of ignition attempts the voltage on this capacitor associated with a START control input is sufficient to cause the controller to be placed into a pre-ignition mode in which the inverter frequency is kept at a high level and prevented from sweeping to a low level to produce the ignition voltage. While limiting the number of ignition attempts, the inverter of this circuit will oscillate and provide an output voltage upon initial energization of the ballast despite the absence of lamps at the lamp terminals. The protection circuit also includes a DC trickle path through a respective filament of each lamp to detect when a lamp which has failed to ignite is replaced. When a lamp is replaced, the trickle path is broken, causing via a set of switches, the capacitor associated with the START input to discharge. This enables the controller to reenter the ignition mode in which the high ignition voltage is produced across the lamps.
U.S. Pat. No. 4,461,980 (Nilssen) discloses a protection circuit in a ballast which disables the inverter within a brief time period of about one second after a lamp is removed from its socket. The protection circuit disables the inverter when the lamp current falls below a prescribed minimum. The circuit includes a thermally responsive bimetal switch which shorts an inverter feedback loop. When lamp current is not flowing, a clamping current flows through a heating filament of the bimetallic switch. In this arrangement, no lamp current flows during the pre-heat and ignition phases, so the bimetallic switch is selected so that it enables a normally functioning lamp to be ignited. This means, however, that upon initial energization of the ballast the inverter is powered-up and the high output voltage appears across the lamp terminals whether a lamp is present or not.
Accordingly, it is the object of the invention to provide a lamp controller with improved lamp status detection and safety circuitry.